A method for fabricating sintered high silica glasses has been disclosed by us in U.S. Pat. No. 4,419,115, (the '115 patent) co-assigned with this, and incorporated herein by reference. The method disclosed in the patent comprises forming a first silica-containing material, forming particles from the first material, redispersing the particles, forming a gel from the redispersed particles, drying the gel, thereby producing a porous body, and thereafter sintering the porous body to form a glass. This method is referred to herein as the double-dispersion method. The double-dispersion process was found to substantially reduce the likelihood of cracking of the drying gel body, as compared to bodies produced by single-dispersion sol-gel processes. As disclosed in the '115 patent, particles typically are formed by fragmentation of dried gel material in a blender.
Another prior art gel method for producing high silica glass uses hydrolysis of one or more metal alkoxides. The alkoxide process has, for instance, been described by S. Sakka in Treatise on Materials Science and Technology, Vol. 22, M. Tomozawa and R. H. Doremus, editors, Academic Press, 1982, pp. 129-167, incorporated herein by reference. After hydrolysis of the alkoxide, the reulting high silica gel is dried, to the extent this is possible, and the resulting material, herein also referred to as a gel, sintered to form glass therefrom.
Dry gel material produced by the process of the '115 patent has, prior to sintering, a pore size distribution exemplified by the distribution shown in FIG. 1. The Figure reveals that a substantial fraction of the total pore volume of the prior art material is contributed by pores of effective diameter larger than about 0.04 .mu.m (40 nm) with a significant peak at about 2 .mu.m (4000 nm). In particular, glass produced by prior art double-dispersion sol-gel processes typically has, prior to sintering, significantly more than 5% of its total pore volume contributed by pores of effective diameter greater than about 0.5 .mu.m. These large pores are difficult to collapse during sintering, requiring a relatively high sintering temperature, and may result in glass defects.
On the other hand, gel material produced by the prior art monolithic (i.e., not comprising comminution of the first material) alkoxide process typically has, prior to sintering, very small pores. In particular, such material typically has a pore size distribution such that more than 90% of the total pore volume is contributed by pores of effective diameter substantially less than about 0.01 .mu.m. This lack of larger pores makes the migration of fluid out of, or into, the body difficult, resulting in disadvantageous processing characteristics of such material. For instance, due to the lack of larger pores, it is virtually impossible to completely dry the gel.
Due to the potential of sol-gel processes for economically producing high quality high silica glass, a process that can produce glass of improved quality at a lower sintering temperature is of considerable interest. This application discloses such a process.